The present invention relates to a release paper. More particularly, the present invention relates to a release paper which comprises a release layer of a specified 4-methyl-1-pentene resin and which can be appropriately used in, for example, production of a synthetic leather.
Generally, synthetic leathers are produced by first coating a release paper having a relief pattern on its surface with a base resin such as polyurethane resin in liquid form, subsequently effecting drying to thereby obtain a resin layer on the release paper, thereafter bonding a base material such as cloth to the resin layer, and finally removing the release paper. The surface of thus produced synthetic leathers is in such a condition that the relief pattern of the surface of release paper has been transferred thereto.
The release paper for use in the above production of synthetic leathers must have excellent heat resistance and release capability. Therefore, in the release paper, a 4-methyl-1-pentene polymer whose melting point is as high as about 220 to 240xc2x0 C. is used as a principal component of a release layer formed on a base such as paper.
For example, a paper base overlaid with poly-4-methyl-1-pentene by extrusion laminating is used as the release paper for synthetic leather. In the production of this release paper for synthetic leather, the release layer is embossed so that desired pattern is provided on the surface of the release layer. The intended synthetic leather can be produced by casting a resin for synthetic leather over the surface of the release layer and hardening the resin.
A release paper for synthetic leather production comprising a paper base laminated with a release layer constituted of a 4-methyl-1-pentene polymer composition comprising 80 to 97 parts by weight of 4-methyl-1-pentene polymer (A) and 3 to 20 parts by weight of ethylene/ethyl acrylate copolymer (B), was proposed in Japanese Patent Laid-open Publication No. 8(1996)-25580. According to this publication, the release layer of the release paper has excellent heat resistance and wettability, so that the release paper can be appropriately used even in the production of a synthetic leather from a water soluble or water dispersible resin.
There has been such a problem that, when the release paper is repeatedly used in the production of a synthetic leather or the like, the release paper curls into cylindrical form. Upon curling of the release paper, the work efficiency is poor in the reuse of the release paper, and, when forced elimination of curling is intended, the surface of the release paper may suffer from flaw or microcracking to thereby cause the reuse thereof to be difficult. The greater the thickness of the release layer of the release paper, the more serious the tendency toward these problems.
Therefore, there has been a strong demand for the development of a repeatedly usable release paper which is excellent in heat resistance and exhibits less curling tendency in repeated use and which exhibits a high bonding strength between resin and paper.
It is an object of the present invention to provide a release paper which is excellent in heat resistance and exhibits less curling tendency in repeated use, which exhibits a high bonding strength between resin and paper and is excellent in durability, and which is suitable for use in, for example, production of a synthetic leather.
The release paper of the present invention comprises:
a paper base laminated with a release layer,
the release layer comprising a 4-methyl-1-pentene resin being a copolymer of 4-methyl-1-pentene and an xcex1-olefin other than 4-methyl-1-pentene wherein the xcex1-olefin is contained in a proportion of 6 to 8% by weight.
With respect to the release paper of the present invention, it is preferred that the xcex1-olefin have 10 to 20 carbon atoms. Preferably, the release paper is for use in production of a synthetic leather.
The present invention will be described in detail below.
The release paper of the present invention comprises a paper base laminated with a release layer. Only one side of the paper base may be laminated with the release layer. Alternatively, both sides of the paper base may be laminated with the release layer.
 less than Release Layer greater than 
The release layer as a constituent of the release paper of the present invention comprises a 4-methyl-1-pentene resin being a copolymer of 4-methyl-1-pentene and an xcex1-olefin other than 4-methyl-1-pentene wherein the xcex1-olefin is contained in a proportion of 6 to 8% by weight.
The xcex1-olefin other than 4-methyl-1-pentene for constituting the 4-methyl-1-pentene resin can be, for example, any of xcex1-olefins having 2 to 20 carbon atoms, such as ethylene, propylene, 1-butene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicocene. At least one of these can be employed. Among them, an xcex1-olefin having 10 to 20 carbon atoms is preferred. In particular, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicocene are especially preferred.
It is preferred that the 4-methyl-1-pentene resin be a copolymer of 4-methyl-1-pentene and an xcex1-olefin having 2 to 20 carbon atoms, especially 10 to 20 carbon atoms, from the viewpoint that the obtained release paper is highly flexible and is excellent in heat resistance.
In the 4-methyl-1-pentene resin for constituting the release layer in the present invention, the xcex1-olefin is contained in a proportion of 6 to 8% by weight, preferably 6 to 7% by weight.
When the xcex1-olefin content of the 4-methyl-1-pentene resin is as low as less than 6% by weight, the crystallinity and orientation coefficient of the 4-methyl-1-pentene resin tend to exhibit unfavorably large values. When the release paper including the release layer constituted of such 4-methyl-1-pentene resin is used in the production of a synthetic leather, the release layer may unfavorably be shrunk by heating in the process for producing the synthetic leather to thereby cause the release paper to curl.
On the other hand, when the xcex1-olefin content of the 4-methyl-1-pentene resin is as high as more than 8% by weight, the heat resistance of the release layer resin may be unsatisfactory. When the release paper including the release layer constituted of such resin is used in the production of a synthetic leather, the embossed pattern provided on the release layer may unfavorably be flowed by heating in the process for producing the synthetic leather to thereby cause the repeated use of the release paper to be infeasible.
When the xcex1-olefin content of the 4-methyl-1-pentene resin is in the range of 6 to 8% by weight, these problems can favorably be avoided to thereby enable the repeated use of the release paper.
With respect to the 4-methyl-1-pentene resin for use in the present invention, it is preferred that the melt flow rate (MFR) thereof as measured under a load of 5.0 kg at 260xc2x0 C. according to ASTM D1238 be in the range of 1 to 400 g/10 min, especially 20 to 300 g/10 min, and still especially 50 to 200 g/10 min. The use of such 4-methyl-1-pentene resin is preferred from the viewpoint that laminating can be efficiently conducted at a high speed to thereby enhance the productivity of release paper, that a uniform thickness can be realized and the release layer can be free from cracking, and that a release paper which is excellent in the bonding strength between the release layer and the paper base layer can be easily produced.
In the present invention, the 4-methyl-1-pentene resin for constituting the release layer can be produced by copolymerizing 4-methyl-1-pentene and at least one of the above xcex1-olefins according to known processes.
In the present invention, the 4-methyl-1-pentene resin may contain various additives customarily incorporated in polyolefins according to necessity in an amount not detrimental to the object of the present invention. Also, the 4-methyl-1-pentene resin may contain a small amount of resins other than the 4-methyl-1-pentene resin. Examples of the additives include a silane coupling agent, a weathering stabilizer, a thermal stabilizer, a slip agent, a nucleating agent, a pigment and a dye.
The thickness of the release layer of the 4-methyl-1-pentene resin, although not particularly limited, is preferably in the range of generally 5 to 200 xcexcm, especially 20 to 100 xcexcm. When the thickness of the release layer is in these preferable ranges, there may be obtained a satisfactory release property to a resin, such as the resin for synthetic leather, and, even if the smoothness of paper base is poor, the release layer can have smooth surface with the result that a synthetic leather of good surface condition can be obtained by the use of the release paper.
Thus formed release layer is excellent in the adherence to a paper base and in heat resistance, and exhibits a low resin orientation and a low crystallinity. The release paper of the present invention having such release layer is characterized by low shrinkage when exposed to heat in the production of a synthetic leather, less curling tendency upon repeated use, less pattern disappearing tendency, high bonding strength and prolonged durability.
 less than Paper Base greater than 
In the present invention, a kraft paper, a wood free paper, a simili paper, an art paper, a coated paper or the like is used as the paper base. The thickness of the paper base, although not particularly limited, is preferably in the range of generally about 50 to 1000 xcexcm, especially 100 to 350 xcexcm.
 less than Release Paper greater than 
The release paper of the present invention comprises the above paper base laminated with the release layer of the 4-methyl-1-pentene resin. Only one side of the paper base maybe laminated with the release layer. Alternatively, both sides of the paper base may be laminated with the release layer. The release paper of the present invention, although may be produced by any method, is preferably produced by laminating the paper base with the resin for constituting the release layer according to the extrusion laminating method. Further, the release paper of the present invention may have embossed pattern. The embossed pattern may be provided during the laminating of the paper base with the release layer or after the laminating. The embossed pattern can appropriately be provided by the use of customary embossing techniques.
According to necessity, the mixing of 4-methyl-1-pentene resin for constituting the release layer and optionally incorporated various additives can be performed by conventional methods using a mixer or a kneader. The employable mixer can be, for example, a V blender, a ribbon blender, a Henschel mixer or a tumbler blender. The employable kneader can be, for example, a single-screw extruder, a double-screw extruder, a kneader or a Banbury mixer.
The release paper of the present invention is preferably prepared by laminating the paper base with the release layer. In particular, it is preferred that the paper base be overlaid with the 4-methyl-1-pentene resin for constituting the release layer by extrusion lamination molding. The extrusion lamination molding can be accomplished by, for example, extruding the 4-methyl-1-pentene resin in molten form on the paper base by extrusion coating by means of customary extrusion laminators such as a single extrusion laminator or a coextrusion laminator. By this manner, the paper base can be laminated with the 4-methyl-1-pentene resin. The release paper obtained by laminating the paper base with the release layer by the extrusion lamination molding exhibits especially excellent bonding strength to thereby exhibit less tendency to invite peeling or other drawbacks even in the repeated use as the release paper for synthetic leather production or the like. Hence, the release paper ensures excellent durability.
The release paper of the present invention, by virtue of the structure wherein the paper base is laminated with the release layer constituted of the 4-methyl-1-pentene resin having an xcex1-olefin content of 6 to 8% by weight, is characterized by reduced orientation of the release layer resin, low crystallinity thereof, less shrinkage when exposed to heating in the use in synthetic leather production or the like, satisfactory release property, excellent heat resistance, reduced curling even after repeated use and excellent durability.
The above release paper of the present invention is suitable for use in the production of a synthetic leather.
The present invention enables providing a release paper which is excellent in heat resistance and exhibits less curling tendency in repeated use, which exhibits a high bonding strength between resin and paper, and is excellent in durability, and which is suitable for use in, for example, production of a synthetic leather.